Magnetism and Electromagnetism

A magnet is an object that attracts magnetic materials such as iron, and that has two poles — a north pole and a south pole. A basic rule of magnetism is that like poles repel and unlike poles attract: two north poles (or two south poles) push apart, while a north pole and a south pole pull together. The region around a magnet is special, because the magnet's influence is felt there even without contact. This region around a magnet within which its magnetic force can be detected is called the magnetic field.

We cannot see a magnetic field directly, but we can map it using magnetic field lines (also called lines of force). A magnetic field line shows the direction of the magnetic field at each point — it is the path a tiny north pole would follow. By convention, field lines come out of the north pole and go into the south pole outside the magnet (and run from south to north inside it), forming closed loops. The pattern of field lines can be revealed by sprinkling iron filings around a magnet, which line up along the field, or by using a small compass to trace the directions.

Magnetic field lines have several important properties. First, they are closed, continuous loops that run from the north pole to the south pole outside the magnet. Second, the closeness of the lines shows the strength of the field — where the lines are crowded together (close), the field is strong (as near the poles), and where they are far apart, the field is weak. Third, two magnetic field lines never cross each other, because at any point the field has only one direction.

The Earth itself behaves like a giant magnet, with its own magnetic field, which is why a freely suspended magnet (a compass needle) always points roughly north–south — the basis of the magnetic compass used for navigation. Understanding the magnetic field and how field lines represent its direction and strength gives us the language to describe magnetism, and prepares us to study the deep connection between electricity and magnetism that follows — for an electric current also produces a magnetic field.

For a long time, electricity and magnetism were thought to be completely separate. Then, in a famous discovery, the scientist Hans Christian Oersted found that they are deeply connected. In Oersted's experiment, a compass needle was placed near a wire carrying an electric current. When the current was switched on, the compass needle was deflected (it turned aside); when the current was switched off, the needle returned to its usual north–south position. This showed that an electric current produces a magnetic field around it — this is the magnetic effect of electric current.

Oersted's discovery means that a current-carrying wire behaves like a magnet, with its own magnetic field surrounding it. The magnetic field lines around a straight current-carrying wire are in the form of concentric circles centred on the wire. The strength of this magnetic field is greater when the current is larger and greater nearer the wire, becoming weaker farther away. So electricity and magnetism are linked: wherever a current flows, a magnetic field is produced.

To find the direction of the magnetic field around a straight current-carrying wire, we use the right-hand thumb rule. Imagine grasping the wire with your right hand so that the thumb points in the direction of the current; then the curled fingers point in the direction of the magnetic field lines around the wire. This simple rule lets us work out which way the circular field lines go for any direction of current.

This connection between electricity and magnetism is enormously important, because it means we can produce and control magnetism using electric current. By passing current through wires, we can create magnetic fields at will, switch them on and off, and make them as strong as we like by increasing the current. This is the basis of electromagnets, electric motors, generators, and transformers — devices that have transformed the modern world. Oersted's experiment, showing that a current produces a magnetic field, is therefore the starting point for the whole study of electromagnetism that follows.

We have seen that a straight current-carrying wire produces a magnetic field. If we wind the wire into a coil of many turns, the magnetic effects add up and become much stronger. A coil of many circular turns of insulated wire wound in the shape of a cylinder is called a solenoid. When a current flows through a solenoid, it produces a magnetic field, and remarkably, the magnetic field of a current-carrying solenoid is just like that of a bar magnet — it has a north pole at one end and a south pole at the other, with a strong, uniform field inside.

The great advantage of a solenoid is that, by placing a piece of soft iron inside it, we can make a very strong magnet that can be switched on and off. An electromagnet is a magnet made by winding a coil of insulated wire around a soft iron core and passing a current through it. While the current flows, the soft iron core becomes strongly magnetised and behaves as a powerful magnet; when the current is switched off, it loses almost all its magnetism. So an electromagnet is a temporary magnet whose magnetism can be controlled by the current — a huge advantage over permanent magnets.

The strength of an electromagnet can be increased in several ways: by increasing the current flowing through the coil, by increasing the number of turns in the coil, and by using a soft iron core (which greatly strengthens the magnetism). These factors let us design electromagnets of just the strength we need, from small ones in devices to gigantic ones in industry. Soft iron is used for the core (rather than steel) precisely because it magnetises strongly when the current is on and demagnetises quickly when it is off.

Electromagnets have many important uses. Huge electromagnets fitted to cranes are used to lift and move heavy loads of iron and steel scrap in factories and junkyards — the load is picked up when the current is on and dropped when it is switched off. Electromagnets are also used in electric bells, telephones, loudspeakers, and many machines, and very powerful ones are used in MRI machines in hospitals. Because their magnetism can be switched on and off and made very strong, electromagnets are extremely useful, and the solenoid that produces this controllable magnetism is a key device in electromagnetism.

We know that a current-carrying wire produces a magnetic field, and that a magnet has a magnetic field. What happens if we place a current-carrying wire in a magnetic field? The two magnetic fields interact, and the result is that a current-carrying conductor placed in a magnetic field experiences a force. This force can push the wire, and it is the basis of one of the most important machines in the modern world — the electric motor. The size of the force is greater when the current is larger and when the magnetic field is stronger.

To find the direction of this force, we use Fleming's left-hand rule. Stretch the thumb, the first finger, and the second finger of the left hand so that they are at right angles to one another. If the first finger points in the direction of the magnetic field (from north to south) and the second (middle) finger points in the direction of the current, then the thumb points in the direction of the force (the direction in which the conductor moves). This rule lets us predict which way a current-carrying wire will be pushed in a magnetic field.

This force on a current-carrying conductor is used in the electric motor, a device that converts electrical energy into mechanical (kinetic) energy — that is, it uses electricity to produce motion. Electric motors are found in fans, mixers, washing machines, pumps, electric vehicles, and countless other machines. The motor takes electrical energy from a supply and makes a shaft rotate, doing useful mechanical work.

A simple DC motor consists of a coil of wire (the armature) placed between the poles of a magnet, connected to a battery through a device called a commutator and brushes. When current flows through the coil in the magnetic field, the two sides of the coil experience forces in opposite directions (by Fleming's left-hand rule), which make the coil rotate. The commutator is a split ring that reverses the direction of the current in the coil every half turn, so that the coil keeps rotating in the same direction continuously. In this way the motor turns electrical energy into continuous rotational motion. So the force on a current-carrying conductor in a magnetic field, with its direction given by Fleming's left-hand rule, is harnessed in the electric motor to drive the machines of everyday life.

We have seen that a current produces a magnetic field. The scientist Michael Faraday discovered the reverse: that a changing magnetic field can produce an electric current. This process — producing an electric current in a coil by changing the magnetic field through it — is called electromagnetic induction, and the current produced is an induced current. In Faraday's experiment, when a magnet is moved in and out of a coil, a current is induced in the coil (shown by a connected meter); the current flows only while the magnet is moving, and stops when the magnet is still. A faster movement or a stronger magnet gives a larger induced current.

The direction of the induced current is governed by Lenz's law (which we study conceptually): the induced current always flows in a direction such that it opposes the change that produced it. In other words, nature "resists" the change in the magnetic field. The key idea to remember is that electromagnetic induction needs a changing magnetic field — simply holding a magnet still near a coil produces no current; the field through the coil must be changing.

Electromagnetic induction is the principle behind the electric generator, a device that converts mechanical (kinetic) energy into electrical energy — the opposite of a motor. In an AC generator, a coil is rotated in a magnetic field (by some mechanical means, such as falling water, steam, or wind). As the coil turns, the magnetic field through it keeps changing, so an induced current flows in it — this current is alternating current (AC), which changes direction periodically. The direction of the induced current in a generator can be found using Fleming's right-hand rule (in contrast to the motor's left-hand rule). Generators in power stations produce the electricity we use.

Another vital device based on induction is the transformer, which is used to change the voltage of an alternating current. A step-up transformer increases the voltage, and a step-down transformer decreases the voltage. A transformer has two coils — a primary and a secondary — wound on an iron core, and it works by induction between them. The voltage change depends on the ratio of the number of turns in the two coils, given by Vₚ / Vₛ = Nₚ / Nₛ (the ratio of primary to secondary voltage equals the ratio of primary to secondary turns). Transformers are essential for transmitting electricity efficiently over long distances: the voltage is stepped up for transmission and stepped down again for safe use in homes. So electromagnetic induction — the production of current by a changing magnetic field — gives us generators and transformers, completing the remarkable two-way link between electricity and magnetism and concluding our study of Grade 8 Physics.